You're right about the ear, I'm working on it, you may as well have replied to one of my other threads on which I was asking about training programs. But in the meantime, why should I not try to sharpen my arsenal and learn more about progressions, you know ?

I guess I'm a little confused. I'm not exactly sure what it is you're trying to do here, because learning chord progressions and improvising chords don't really seem that connected to me.

Part of the issue is that, let's say the bassist and drummer are already jamming away, and I want to join: chances are good that the bassist is already playing with an implied chord progression - he's merely arpeggiating and implying the chords rather than strumming them.

If not, if he's playing primarily melodically, then what I want to do is harmonize his melody.

A lot of backing tracks you find online don't really have a chord progression. The bass is essentially arpeggiating out one chord. Sometimes two.

If he's doing one chord, it can be hard to put a progression on top of what he's doing since he's not going anywhere. eg, if he's doing a riff bassed on A C and E (an Am chord) then I might be able to play an Am or a C major over it, but beyond that it gets tricky.

eg, the most obvious places for me to go are a Dm and an E chord, right? But that Dm is D A F, so you've got a pretty significant clash between the E and the F, and a smaller not trivial between the C and the D. Alternatively, I could play an E major chord, but now we're dealing with the clash between the G# and the A, and the B and the C. It's going to be hard to make that all sound good.

The best you're likely to do is to copy his chord progression and then mix up your choices from within the chord families. eg, in key of C, when he's playing a C major, you play a C major, E minor, or A minor. He plays an F and you can play a Dm or an F. He plays a G you can play a G or a B diminished.

It turns out there are lots of different ways you can sub different chords to make interesting things happen (C6/9 for C, G13 for G7 for example) but this is really diving down the jazz rabbit hole. And, in one of those things that's hard to understand, it really doesn't work that well when your starting point is an academic one, rather than hearing the sound in your head you want to play.

Ultimately all it takes is a solid understanding of functional harmony and a solid ear.

Really it's quite easy to improvise a simple progression if you have the vocabulary under your belt. I do it all the time when I play acoustic sets with my band.

What really impresses me is how my singer comes up with verses/choruses (including lyrics) and basically feels her way through an entire song structure in real time. It blows my mind how good it sounds. Now, her lyrics aren't anything groundbreaking, but they sound good still...

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Only play what you hear. If you donít hear anything, donít play anything.
-Chick Corea